Broad or bimodal molecular weight distribution polymer compositions are compositions that typically include one or more high molecular weight polymers and one or more low molecular weight polymers. In bimodal molecular weight distribution polymer compositions, the weight fraction of the high molecular weight (“HMW”) polymer typically ranges from, for example, 0.10 to 0.90 for applications requiring broad molecular weight distribution polymers. The relative amount of HMW polymer in the polymer composition can influence the rheological properties of the composition. One such measurable rheological property of bimodal polymer compositions is its flow index (“FI”, or I21, measured at 190° C., 21.6 kg according to ASTM D-1238). The I21 of the bimodal polymer composition, in one embodiment, possesses an I21 that is between 2 and 100 dg/min. This range represents a balance between processability (desiring relatively high I21) on the one hand, and product (film, etc.) toughness (desiring relatively low I21) on the other hand. Hence, it is necessary to control polymer composition I21 in the polymerization reactor.
One desirable method of producing bimodal polymer compositions is through the use of a bimetallic catalyst composition in a single reactor such as disclosed in, for example, U.S. Pat. No. 5,882,750 and U.S. Pat. No. 6,274,684. Such compositions incorporate at least two, preferably two, metal centers, both of which may be the same or different metal with similar or differing coordination spheres, patterns of substitution at the metal center or ligands bound to the metal center. Typically, one of the metal centers produces a low molecular weight (“LMW”) polymer while the other produces a HMW polymer in the single polymerization reactor, and desirably, although not necessarily, simultaneously. U.S. Pat. No. 5,525,678 to Mink et al. discloses the use of water and carbon dioxide in controlling the I21 using one such bimetallic catalyst composition. WO 02/46246 to Mawson et al.; U.S. Pat. Nos. 6,420,474 and 6,569,963 to Nowlin et al. disclose the addition of an additional catalyst to adjust the relative amounts of HMW and LMW polymers in a polymer composition. However, the products resulting therefrom, such as bimodal resins used to make films, may still suffer from gel formation, the reaction process itself may be subject to fouling, which causes an undesirable need to shut down the polymerization reactor, and further, the procedure of adding a catalyst component can add cost and complexity to the process.
What is needed is an improved method of controlling I21 in the manufacture of bimodal polymer compositions when using bimetallic catalyst compositions. While many types of compounds have been disclosed for use in polymerization reactors, for example in WO 01/00691; U.S. Pat. No. 6,596,824; U.S. Pat. No. 6,417,299; U.S. Pat. No. 6,417,298; U.S. Pat. No. 6,313,236; U.S. Pat. No. 5,733,988, U.S. Pat. No. 5,625,012; U.S. Pat. No. 5,244,987; U.S. Pat. No. 5,200,502; U.S. Pat. No. 5,194,526; U.S. Pat. No. 5,066,736; U.S. Pat. No. 4,855,370; and EP 1 159 311, EP 0 630 910 and EP 0 315 192; and Soviet Union Patent No. 1 249 025, there is still the problem of effective I21 control in the manufacture of bimodal polymer compositions when using bimetallic catalyst compositions. The inventors have unexpectedly found that there exists a broad class of compounds useful as I21 control agents, or “split” control agents, that also have other advantages as described herein.